Sir, Sir, Sir... What am I going to do with you??? I never claimed to be a God in any way, shape, or form. And I don´t remeber telling you about a problem like this being caused by the ECU either. If a check engine light comes on after a thermostat install the first thing I would do is make sure you got the right temperature thermostat. If you got one that is cooler than what is sugested than you vehicle won´t go into what is called closed loop operation where it take readings from the oxegen sensor to compensate for variations in the air fuel mixture. It will cause the car to run rich, and some emissions equipment might not operate correctly. The best thing to do when you get a check engine light is to run a diagnostic test to see what codes come up from the computer. It is easy enough to do yourself, but Auto Zone also has the scanners and will check your codes for free. Just make sure they give you a list of what they came up with. Another thing could be that you knocked a wire loose while you were taking the thermostat out, or maybe a totaly unrelated component went bad. Thats why you need to see what codes the ECU has stored before you try anything else. Clear the codes only after you have recorded them on a piece of paper, then drive around for a while and check again to see if any are repeated.
Could be an air bubble and the sending unit for the temp to the computer isn´t reading anything, then going into closed loop the info is missing and the ECU thinks it´s a fubared sensor? Go for obvious first make sure anything unhooked was hooked back up.